[BUG] sysbench flaky

[nautaa]

Describe the bug
A clear and concise description of what the bug is.

add some observer.log to debug at first.

Run cd test/sysbench
sysbench 1.0.20 (using system LuaJIT 2.1.0-beta3)

Running the test with following options:
Number of threads: 10
Initializing random number generator from current time


Initializing worker threads...

Threads started!

FATAL: mysql_store_result() returned error 2013 (Lost connection to MySQL server during query)
(last message repeated 1 times)
FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'SELECT * FROM sbtest1 WHERE k=4[7](https://github.com/oceanbase/miniob/actions/runs/13941989095/job/39020571510?pr=528#step:9:8)77'
FATAL: mysql_store_result() returned error 2013 (Lost connection to MySQL server during query)
(last message repeated 1 times)
FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'SELECT * FROM sbtest1 WHERE k=4995'
FATAL: mysql_store_result() returned error 2013 (Lost connection to MySQL server during query)
(last message repeated 1 times)
FATAL: `thread_run' function failed: ./miniob_select.lua:31: SQL error, errno = 2013, state = 'HY000': Lost connection to MySQL server during query
FATAL: `thread_run' function failed: ./miniob_select.lua:31: SQL error, errno = 2013, state = 'HY000': Lost connection to MySQL server during query
FATAL: `thread_run' function failed: ./miniob_select.lua:31: SQL error, errno = 2013, state = 'HY000': Lost connection to MySQL server during query
(last message repeated 2 times)
FATAL: mysql_store_result() returned error 2013 (Lost connection to MySQL server during query)
(last message repeated 1 times)
FATAL: `thread_run' function failed: ./miniob_select.lua:31: SQL error, errno = 2013, state = 'HY000': Lost connection to MySQL server during query
(last message repeated 2 times)
FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'SELECT * FROM sbtest1 WHERE k=537[8](https://github.com/oceanbase/miniob/actions/runs/13941989095/job/39020571510?pr=528#step:9:9)'
FATAL: `thread_run' function failed: ./miniob_select.lua:31: SQL error, errno = 2013, state = 'HY000': Lost connection to MySQL server during query
Error: Process completed with exit code 1.

ref: https://github.com/oceanbase/miniob/actions/runs/13941989095/job/39020571510?pr=528

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